U.S. Pat. No. Re. 24,906 (Ulrich) concerns pressure-sensitive adhesive copolymer of about 88-97 parts of acrylic acid ester of non-tertiary alcohol, the alkyl groups of which have an average of 4-12 carbon atoms in the alkyl group, and correspondingly about 12-3 parts by weight of at least one modifying copolymerizable monomer such as acrylic acid, itaconic acid or acrylamide. A tape coated with the copolymer exhibits excellent adhesion and holding power, and the adhesive coating experiences no observable deterioration even after the tape has been stored for a number of years.
While the tape coatings of the Ulrich patent are applied from solution, U.S. Pat. No. 3,661,618 (Brookman et al.) concerns a process wherein a solvent-free coating primarily comprising an acrylic ester monomer of Ulrich is polymerized to a tacky state by being subjected to a beam of high energy electrons. The coating preferably includes a small amount of a polyfunctional acrylate or methacrylate such as diethylene glycol diacrylate which crosslinks the acrylic acid ester, thus improving the cohesive strength of the tacky coating. The use of an electron beam not only eliminates the need for a solvent but also eliminates problems due to evaporation of the solvent.
U.S. Pat. No. 4,181,752 (Martens et al.) concerns pressure-sensitive adhesives made from the same liquid monomers as in the Ulrich patent. Solvent-free coatings of those monomers are subjected to radiation having wavelengths of 3000-4000 A at a rate of 0.1 to 7 milliwatts per square centimeter, while controlling the amount of radiation of shorter wavelengths to not more than about 10% of the amount of energy in the 3000-4000 A range. As in the Brookman patent, the Martens process eliminates solvent-evaporation problems.
The Martens patent teaches that the pressure-sensitive adhesive layer may be crosslinked, particularly where it is desired to increase the cohesive strength of the adhesive without unduly affecting its compliance. This can be achieved by utilizing a photoactive crosslinking agent in conjunction with the photoinitiator. Preferred as the photoactive crosslinking agent are certain chromophore-substituted vinylhalomethyl-s-triazines such as 2,4-bis(trichloromethyl)-6-p-methoxystyryl-s-triazine which is hereinafter called "MOST". Because these crosslinkers contain chlorine, they can have a corrosive effect and thus are considered unsuitable for corrosion-sensitive applications such as general electrical-insulating uses. Because it can be difficult to match the peak energy-absorbing wavelength of the photoactive crosslinking agent to that of the primary photoinitiator, it may be necessary to use an ultraviolet source which is not optimum for either of these. Because photoactive triazines such as MOST are highly absorptive of ultraviolet radiation, it has not been considered to be feasible to polymerize coatings which are greater than 0.25 mm in thickness. Also MOST imparts a yellow color which is undesirable for some uses.
In the periodical Chemtech, September 1974, pages 539-543, an article entitled "Radiation Polymerization for Pressure-Sensitive Adhesives" by Dowbenko et al. concerns polymerization of coatings of liquid monomers of the Ulrich patent by electron beam or ultraviolet radiation. The Dowbenko article states that difunctional "monomers of the ethylene glycol diacrylate type can also be used in some cases to increase certain properties of the adhesive, probably by crosslinking" (page 540).
U.S. Pat. No. 4,150,170 (Lazear et al.) concerns the use of ultraviolet radiation to polymerize solvent-free monomeric compositions to a pressure-sensitive adhesive state. The novelty resides in a unique combination of photoinitiators, and it is said that these may be used with any of the compositions previously used in ultraviolet curable systems. Among a host of materials which are said to have been used in previous systems are various acrylic monomers, and with these may be used polyfunctional materials such as ethylene glycol diacrylate. Of these, the examples employ only hexanediol diacrylate.
Of pressure-sensitive adhesive tapes specifically identified in the above-discussed prior art, the tape affording the highest cohesive strength while still having good resistance to peelback is believed to be tape of Example 27 of the Martens patent. Its adhesive was obtained from a mixture of 98 parts isooctyl acrylate, 2 parts acrylic acid, about 0.25 weight percent of benzoin ethyl ether and 0.15 weight percent of MOST. The reported peel adhesion from glass was greater than 0.8 kilogram per centimeter and shear strength exceeded 10,000 minutes [measured by pressing a 1/2-inch square (1.25 cm square) area of tape against a vertical stainless steel plate and hanging a 1-kg weight on the tape].
There has long been a need for a pressure-sensitive adhesive tape which could be made without evolving solvent and would have resistance to peel forces and a cohesive strength at least as good as those of the MOST-based tape of the Martens patent while being less corrosive.